One of the most important catalysts in common usage today is hopcalite, a mixed oxide of copper and manganese that is used as the catalyst of choice for the oxidation of carbon monoxide in breathing apparatus in both military and civil applications. Hopcalite catalysts are also used in other applications, for example removal of ozone from various off-gas emissions by conversion to oxygen and removal of volatile organic compounds from, for example, fuel cell feedstocks.
It is generally recognized that active hopcalite catalysts are prepared by coprecipitation of a basic carbonate from a solution of the nitrates, and that the best catalysts are derived from amorphous precursors. However, as with all catalysts that are in current commercial usage, there is a need to improve the formulation to achieve improved activity so that the catalyst lifetime can be extended, which is particularly important for hopcalite since it is used in breathing apparatus, smoke hoods and other personal protection applications.
In recent years there has been extensive interest in the use of supercritical fluids as antisolvents for precipitation. The particular advantage of this technique is that it permits much higher diffusivities, up to two orders of magnitude greater, when compared with non-supercritical solvents. This fast diffusion can produce supersaturation immediately prior to precipitation and thus leads to the formation of very small particles not accessible by standard catalyst preparation methods.
To date, there has been significant interest in the use of supercritical antisolvent precipitation for the preparation of micro- and nano-particles and the technique has been used to synthesize a range of materials including: polymers, explosives, pharmaceutical chemicals, superconductors and some catalysts. With respect to catalysts the method has been used to prepare a range of single oxides that can be used as either the active phase or as a support material. However, there have been no reports of the method being used to successfully prepare mixed-metal oxides for use as catalysts, as phase separation was thought to be a problem.
Mixed-metal oxides make up, by far, the greatest number of catalysts or catalyst precursors currently used today and it is, therefore, desirable to find a method to prepare mixed-metal oxide catalysts by supercritical antisolvent precipitation.